1. Field of the Invention
The present invention relates to a reading apparatus that reads an image.
2. Description of the Related Art
Electronic devices are generally susceptible to the influence of electromagnetic waves emitted by other nearby devices, natural phenomena such as lightning, static electricity generated during user operations, and so on, which can cause drops in performance, erroneous operations, crashes, the loss of recorded content, and so on if no measures are taken. Therefore, various types of electronic devices are required to meet certain EMS (Electro Magnetic Susceptibility) standards established by government regulations, self-imposed rules created by industry groups, and so on. It is necessary for the designers of electronic devices to balance EMS regulations with device performance, and thus measures involving electric circuit-based techniques that eliminate the influence of external electromagnetic waves, configuration-related techniques that surround electric circuits with metallic mechanical housings to inhibit the penetration of electromagnetic waves, and so on have been taken.
An image reading apparatus, which is an example of an electronic device, is primarily configured of an image sensor, an AD converter, and a digital circuit including logic circuits, memories, a CPU, and so on. Because the respective semiconductor processes differ from one another, it is rare for these elements to be fit into a single integrated chip. It is thus often the case that the elements are wired upon a board with a certain degree of distance therebetween. Meanwhile, particularly with image reading apparatuses that read original documents by scanning those documents in the sub scanning direction with a one-dimensional image sensor, it is often the case that the board on which the image sensor is provided and the board on which the CPU is provided are connected by a long wire such as a FFC (flexible flat cable). For example, an FFC that is several tens of cm to nearly 1 m is used in an image reading apparatus that reads A4-sized original documents.
In the case where the board is divided into multiple sections and electric signals are exchanged between separate boards through cables, impedance generally arises among the respective board ground potentials, which makes it impossible to commonalize the ground potentials at high frequencies. For this reason, the apparatus performs poorly with respect to electromagnetic waves from the exterior, noise, fluctuations in the ground potential, and so on. Meanwhile, the tendency for the cable that connects the boards to act as an antenna and receive electromagnetic waves from the exterior increases.
For such reasons, the image reading apparatus is susceptible to the influence of external electromagnetic waves, static electricity, and so on. Accordingly, if the apparatus has operated erroneously during the reading of an image and there is no configuration for detecting the erroneous operation, the quality of the read image cannot be maintained, the image will be lost, or the like.
Japanese Patent Laid-Open No. 2003-324350 (referred to as “Patent Document 1” hereinafter) discloses a technique for detecting erroneous operations in a PLL of an image reading apparatus. According to Patent Document 1, the number of clocks generated by the PLL among the synchronization signals in a single line is counted; when the result of the counting does not match a set count number, it is determined that the clock has not been properly supplied and a PLL error has occurred, and the operation of the apparatus is stopped.
However, the technique disclosed in Patent Document 1 is a technique that proposes a method for detecting the lock state (synchronization state) of a PLL that has been designed specifically for an image reading apparatus. In other words, Patent Document 1 aims to detect problems and instabilities (out-of-synchronization states) in the operations of the PLL alone, and does not take into consideration sudden operational instabilities occurring in the PLL during image reading, recovery procedures for reading operations, and so on.